On 19.05.21 13:45, Anshuman Khandual wrote:
On 5/18/21 2:48 PM, Muchun Song wrote:
The preparation of supporting freeing vmemmap associated with each
HugeTLB page is ready, so we can support this feature for arm64.
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
arch/arm64/mm/mmu.c | 5 +++++
fs/Kconfig | 2 +-
2 files changed, 6 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 5d37e461c41f..967b01ce468d 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -23,6 +23,7 @@
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/set_memory.h>
+#include <linux/hugetlb.h>
#include <asm/barrier.h>
#include <asm/cputype.h>
@@ -1134,6 +1135,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
pmd_t *pmdp;
WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
+
+ if (is_hugetlb_free_vmemmap_enabled() && !altmap)
+ return vmemmap_populate_basepages(start, end, node, altmap);
Not considering the fact that this will force the kernel to have only
base page size mapping for vmemmap (unless altmap is also requested)
which might reduce the performance, it also enables vmemmap mapping to
be teared down or build up at runtime which could potentially collide
with other kernel page table walkers like ptdump or memory hotremove
operation ! How those possible collisions are protected right now ?
Hi Anshuman,
Memory hotremove is not an issue IIRC. At the time memory is removed,
all huge pages either have been migrated away or dissolved; the vmemmap
is stable.
vmemmap access (accessing the memmap via a virtual address) itself is
not an issue. Manually walking (vmemmap) page tables might behave
differently, not sure if ptdump would require any synchronization.
Does not this vmemmap operation increase latency for HugeTLB usage ?
Should not this runtime enablement also take into account some other
qualifying information apart from potential memory save from struct
page areas. Just wondering.
That's one of the reasons why it explicitly has to be enabled by an admin.
--
Thanks,
David / dhildenb
